Accessing analytes in biofluids for peripheral biochemical monitoring

Printed Organic Transistor-based Biosensors for Non-invasive Sweat Analysis

This review describes recent advances in biosensors for non-invasive human healthcare applications, especially focusing on sweat analysis, along with approaches for fabricating these biosensors based on printed electronics technology. Human sweat contains various kinds of biomarkers. The relationship between a trace amount of sweat biomarkers partially partitioned from blood and diseases has been investigated by omic analysis. Recent progress in wearable or portable biosensors has enabled periodic or continuous monitoring of some sweat biomarkers while supporting the results of the omic analysis. In this review, we particularly focused on a transistor-based biosensor that is highly sensitive in quantitatively detecting the low level of sweat biomarkers. Furthermore, we showed a new approach of flexible hybrid electronics that has been applied to advanced sweat biosensors to realize fully integrated biosensing systems wirelessly connected to a networked IoT system. These technologies are based on uniquely advanced printing techniques that will facilitate mass fabrication of high-performance biosensors at low cost for future smart healthcare.

Physiological mechanisms determining eccrine sweat composition

Purpose

The purpose of this paper is to review the physiological mechanisms determining eccrine sweat composition to assess the utility of sweat as a proxy for blood or as a potential biomarker of human health or nutritional/physiological status.

Methods

This narrative review includes the major sweat electrolytes (sodium, chloride, and potassium), other micronutrients (e.g., calcium, magnesium, iron, copper, zinc, vitamins), metabolites (e.g., glucose, lactate, ammonia, urea, bicarbonate, amino acids, ethanol), and other compounds (e.g., cytokines and cortisol).

Results

Ion membrane transport mechanisms for sodium and chloride are well established, but the mechanisms of secretion and/or reabsorption for most other sweat solutes are still equivocal. Correlations between sweat and blood have not been established for most constituents, with perhaps the exception of ethanol. With respect to sweat diagnostics, it is well accepted that elevated sweat sodium and chloride is a useful screening tool for cystic fibrosis. However, sweat electrolyte concentrations are not predictive of hydration status or sweating rate. Sweat metabolite concentrations are not a reliable biomarker for exercise intensity or other physiological stressors. To date, glucose, cytokine, and cortisol research is too limited to suggest that sweat is a useful surrogate for blood.

Conclusion

Final sweat composition is not only influenced by extracellular solute concentrations, but also mechanisms of secretion and/or reabsorption, sweat flow rate, byproducts of sweat gland metabolism, skin surface contamination, and sebum secretions, among other factors related to methodology. Future research that accounts for these confounding factors is needed to address the existing gaps in the literature.

Electronic supplementary material

The online version of this article (10.1007/s00421-020-04323-7) contains supplementary material, which is available to authorized users.

A wearable freestanding electrochemical sensing system

To render high-fidelity wearable biomarker data, understanding and engineering the information delivery pathway from epidermally retrieved biofluid to a readout unit are critical. By examining the biomarker information delivery pathway and recognizing near-zero strained regions within a microfluidic device, a strain-isolated pathway to preserve biomarker data fidelity is engineered. Accordingly, a generalizable and disposable freestanding electrochemical sensing system (FESS) is devised, which simultaneously facilitates sensing and out-of-plane signal interconnection with the aid of double-sided adhesion. The FESS serves as a foundation to realize a system-level design strategy, addressing the challenges of wearable biosensing, in the presence of motion, and integration with consumer electronics. To this end, a FESS-enabled smartwatch was developed, featuring sweat sampling, electrochemical sensing, and data display/transmission, all within a self-contained wearable platform. The FESS-enabled smartwatch was used to monitor the sweat metabolite profiles of individuals in sedentary and high-intensity exercise settings.

Highly stretchable potentiometric ion sensor based on surface strain redistributed fiber for sweat monitoring

Development of stretchable potentiometric ion sensors has the observable potential for wearable devices to continuously monitoring of electrolytes in body fluids. However, the mechanical mismatch between soft elastomeric substrate and ion-selective electrode components greatly hinders sensor's fabrication and its stretching stability for long-term use. Here, we propose a new strategy to construct a potentiometric ion sensor on a surface strain redistributed elastic fiber (SSRE-fiber) with both high stretchability and high sensing stability. The SSRE-fiber is designed with a unique unilateral bead structure, which significantly changes its surface strain distribution during deformation. Benefit from this platform, the active sensing materials with high Young's modulus fabricated on the unilateral bead region can keep unchanged during stretching (0-200%). Thus, the as-prepared potentiometric sensors (ion-selective electrode and polymer/inorganic salt membrane-coated reference electrode) can perform with stable functions ignoring the stretching of the fiber. This new SSRE-fiber platform paves a way for the design of highly stretchable and stable electrochemical sensor capable of integrating into textiles for wearable biochemical detection applications.

Microneedles for transdermal diagnostics: Recent advances and new horizons

Point-of-care testing (POCT), defined as the test performed at or near a patient, has been evolving into a complement to conventional laboratory diagnosis by continually providing portable, cost-effective, and easy-to-use measurement tools. Among them, microneedle-based POCT devices have gained increasing attention from researchers due to the glorious potential for detecting various analytes in a minimally invasive manner. More recently, a novel synergism between microneedle and wearable technologies is expanding their detection capabilities. Herein, we provide an overview on the progress in microneedle-based transdermal biosensors. It covers all the main aspects of the field, including design philosophy, material selection, and working mechanisms as well as the utility of the devices. We also discuss lessons from the past, challenges of the present, and visions for the future on translation of these state-of-the-art technologies from the bench to the bedside.

Design Framework and Sensing System for Noninvasive Wearable Electroactive Drug Monitoring

Wearable drug monitoring targeting epidermally retrievable biofluids (e.g., sweat) can enable a variety of applications, including drug compliance/abuse monitoring and personalized therapeutic drug dosing. In that regard, voltammetry-based approaches are suitable because they uniquely leverage the electroactive nature of target drug molecules for quantification, eliminating the reliance on the availability of recognition elements. However, to adapt such approaches for the envisioned application, three main challenges must be addressed: (1) constructing a sensitive voltammetric sensing interface with high signal-to-background ratio, (2) decoupling the confounding effect of endogenous electroactive species (naturally present in complex biofluid matrices) and baseline variation, and (3) realizing wireless voltammetric excitation and signal acquisition/transmission. To this end, first, a framework for the quantification of electroactive drugs is presented, which centers on the evaluation and determination of suitable sensing electrodes and characterization of the interference from a panel of physiologically relevant electroactive species. This framework was utilized to establish the design space and operational settings for the development of a coupled sensing system and analytical framework to render sample-to-answer drug readouts in complex biofluid matrices. The presented design framework and sensing system can serve as a basis for future wearable sensor development efforts aiming to monitor electroactive species such as pharmaceutical molecules.

Natural Perspiration Sampling and in Situ Electrochemical Analysis with Hydrogel Micropatches for User-Identifiable and Wireless Chemo/Biosensing

Recent advances in microelectronics, microfluidics, and electrochemical sensing platforms have enabled the development of an emerging class of fully integrated personal health monitoring devices that exploit sweat to noninvasively access biomarker information. Despite such advances, effective sweat sampling remains a significant challenge for reliable biomarker analysis, with many existing methods requiring active stimulation (e.g., iontophoresis, exercise, heat). Natural perspiration offers a suitable alternative as sweat can be collected with minimal effort on the part of the user. To leverage this phenomenon, we devised a thin hydrogel micropatch (THMP), which simultaneously serves as an interface for sweat sampling and a medium for electrochemical sensing. To characterize the performance of the THMP, caffeine and lactate were selected as two representative target molecules. We demonstrated the suitability of the sampling method to track metabolic patterns, as well as to render sample-to-answer biomarker data for personal monitoring (through coupling with an electrochemical sensing system). To inform its potential application, this biomarker sampling and sensing system is incorporated within a distributed terminal-based sensing network, which uniquely capitalizes on the fingertip as a site for simultaneous biomarker data sampling and user identification.

Ultra-simple wearable local sweat volume monitoring patch based on swellable hydrogels

Quantifiably monitoring sweat rate and volume is important to assess the stress level of individuals and/or prevent dehydration, but despite intense research, a convenient, continuous, and low-cost method to monitor sweat rate and total sweat volume loss remains an un-met need. We present here an ultra-simple wearable sensor capable of measuring sweat rate and volume accurately. The device continuously monitors sweat rate by wicking the produced sweat into hydrogels that measurably swell in their physical geometry. The device has been designed as a simple to fabricate, low-cost, disposable patch. This patch exhibits stable and predictable operation over the maximum variable chemistry expected for sweat (pH 4-9 and salinity 0-100 mM NaCl). Preliminary in vivo testing of the patch has been achieved during aerobic exercise, and the sweat rates measured via the patch accurately follow actual sweat rates.

Microfluidics for interrogating live intact tissues

The intricate microarchitecture of tissues - the "tissue microenvironment" - is a strong determinant of tissue function. Microfluidics offers an invaluable tool to precisely stimulate, manipulate, and analyze the tissue microenvironment in live tissues and engineer mass transport around and into small tissue volumes. Such control is critical in clinical studies, especially where tissue samples are scarce, in analytical sensors, where testing smaller amounts of analytes results in faster, more portable sensors, and in biological experiments, where accurate control of the cellular microenvironment is needed. Microfluidics also provides inexpensive multiplexing strategies to address the pressing need to test large quantities of drugs and reagents on a single biopsy specimen, increasing testing accuracy, relevance, and speed while reducing overall diagnostic cost. Here, we review the use of microfluidics to study the physiology and pathophysiology of intact live tissues at sub-millimeter scales. We categorize uses as either in vitro studies - where a piece of an organism must be excised and introduced into the microfluidic device - or in vivo studies - where whole organisms are small enough to be introduced into microchannels or where a microfluidic device is interfaced with a live tissue surface (e.g. the skin or inside an internal organ or tumor) that forms part of an animal larger than the device. These microfluidic systems promise to deliver functional measurements obtained directly on intact tissue - such as the response of tissue to drugs or the analysis of tissue secretions - that cannot be obtained otherwise.

Skin-Friendly Electronics for Acquiring Human Physiological Signatures

Epidermal sensing devices offer great potential for real-time health and fitness monitoring via continuous characterization of the skin for vital morphological, physiological, and metabolic parameters. However, peeling them off can be difficult and sometimes painful especially when these skin-mounted devices are applied on sensitive or wounded regions of skin due to their strong adhesion. A set of biocompatible and water-decomposable "skin-friendly" epidermal electronic devices fabricated on flexible, stretchable, and degradable protein-based substrates are reported. Strong adhesion and easy detachment are achieved concurrently through an environmentally benign, plasticized protein platform offering engineered mechanical properties and water-triggered, on-demand decomposition lifetime (transiency). Human experiments show that multidimensional physiological signals can be measured using these innovative epidermal devices consisting of electro- and biochemical sensing modules and analyzed for important physiological signatures using an artificial neural network. The advances provide unique, versatile capabilities and broader applications for user- and environmentally friendly epidermal devices.

Salivary uric acid is a predictive marker of body fat percentage in adolescents

As saliva showed to be a noninvasive source of markers useful to monitor clinical status, the hypothesis tested was that saliva may provide reliable markers able to predict the body fat accumulation in young subjects. The salivary characteristics of 248 adolescent scholars (119 girls; 14-17 years) of flow rate, pH, phosphorus, urea, and calcium concentrations were assessed in stimulated saliva (colorimetric automated technique). The concentrations of cholesterol, 7-ketocholesterol, 25-hydroxyvitamin D2 and D3, and uric acid (UA) were measured with high-performance liquid chromatography in saliva collected at home (12-hour fast). Physical examination included height, weight, and body fat percentage (%BF) measured using bioelectric impedance to classify groups in below/above the %BF cutoff. Data were evaluated using 2-way analysis of variance and multiple linear regression. No significant difference was found in the levels of 25-hydroxyvitamin D2 and D3, cholesterol, 7-ketocholesterol, phosphorus, calcium, and urea between groups above and below %BF cutoff, and the variation in salivary flow was small. Significant sex and group effects were observed for salivary UA, which was increased in adolecents with higher %BF and in males (compared to females), without sex group interaction (power = 99.8%). Sex showed a significant effect on salivary urea, with lower levels in females. A predictive model was obtained, with salivary UA and sex explaining the variation of %BF (P < .001; power = 84%). Salivary UA showed to be an important marker of body fat accumulation in adolescents, demonstrating the clinical relevance of saliva to detect early changes and to monitor the nutritional status using a noninvasive and accurate method.

Wearable and Implantable Electronics: Moving toward Precision Therapy

Soft wearable and implantable electronic systems have attracted tremendous attention due to their flexibility, conformability, and biocompatibility. Such favorable features are critical for reliably monitoring key biomedical and physiological information (including both biophysical and biochemical signals) and effective treatment and management of specific chronic diseases. Miniaturized, fully integrated self-powered bioelectronic devices that can harvest energy from the human body represent promising and emerging solutions for long-term, intimate, and personalized therapies. In this Perspective, we offer a brief overview of recent advances in wearable/implantable soft electronic devices and their therapeutic applications ranging from drug delivery to tissue regeneration. We also discuss the key opportunities, challenges, and future directions in this important area needed to fulfill the vision of personalized medicine.

Wearable flexible sweat sensors for healthcare monitoring: a review

The state-of-the-art in wearable flexible sensors (WFSs) for sweat analyte detection was investigated. Recent advances show the development of integrated, mechanically flexible and multiplexed sensor systems with on-site circuitry for signal processing and wireless data transmission. When compared with single-analyte sensors, such devices provide an opportunity to more accurately analyse analytes that are dependent on other parameters (such as sweat rate and pH) by improving calibration from in situ real-time analysis, while maintaining a lightweight and wearable design. Important health conditions can be monitored and on-demand regulating drugs can be delivered using integrated wearable systems but require correlation verification between sweat and blood measurements using in vivo validation tests before any clinical application can be considered. Improvements are necessary for device sensitivity, accuracy and repeatability to provide more reliable and personalized continuous measurements. With rapid recent development, it can be concluded that non-invasive WFSs for sweat analysis have only skimmed the surface of their health monitoring potential and further significant advancement is sure to be made in the medical field.

In Situ Sampling and Monitoring Cell-Free DNA of the Epstein-Barr Virus from Dermal Interstitial Fluid Using Wearable Microneedle Patches

Using polymerase chain reaction and genotyping, Epstein-Barr virus cell-free DNA (EBV Cf DNA) was detectable in interstitial fluid (ISF). Microneedles offer a minimally invasive approach to capture such Cf DNA. However, a key challenge of microneedles lies in the ability to specifically isolate biomarkers within a short time. We introduced a hydrogel microneedle patch for rapid and easy capture of EBV Cf DNA from ISF in situ around 15 min, with a maximum capture efficiency of 93.6%. Then, quantification of EBV Cf DNA was achieved by electrochemical recombinase polymerase amplification wearable flexible microfluidics, with a detection limit of 3.7 × 10² copies/μL. Animal tests supported the performance of microneedles for EBV Cf DNA capture. Collectively, these data showed that the microneedle patch may have broad implications for patients with Cf DNA-related disease and cancer metastasis in minimally invasive manners.

Single-Molecule Protein Detection in a Biofluid Using a Quantitative Nanopore Sensor

Protein detection in complex biological fluids has wide-ranging significance across proteomics and molecular medicine. Existing detectors cannot readily distinguish between specific and nonspecific interactions in a heterogeneous solution. Here, we show that this daunting shortcoming can be overcome by using a protein bait-containing biological nanopore in mammalian serum. The capture and release events of a protein analyte by the tethered protein bait occur outside the nanopore and are accompanied by uniform current openings. Conversely, nonspecific pore penetrations by nontarget components of serum, which take place inside the nanopore, are featured by irregular current blockades. As a result of this unique peculiarity of the readout between specific protein captures and nonspecific pore penetration events, our selective sensor can quantitatively sample proteins at single-molecule precision in a manner distinctive from those employed by prevailing methods. Because our sensor can be integrated into nanofluidic devices and coupled with high-throughput technologies, our approach will have a transformative impact in protein identification and quantification in clinical isolates for disease prognostics and diagnostics.

4 Ds in health research-working together toward rapid precision medicine

Patient therapy is based mainly on a combination of diagnosis, suitable monitoring or support devices and drug treatment and is usually employed for a pre‐existing disease condition. Therapy remains predominantly symptom‐based, although it is increasingly clear that individual treatment is possible and beneficial. However, reasonable precision medicine can only be realized with the coordinated use of diagnostics, devices and drugs in combination with extensive databases (4Ds), an approach that has not yet found sufficient implementation. The practical combination of 4Ds in health care is progressing, but several obstacles still hamper their extended use in precision medicine.

A wearable electrofluidic actuation system

We report a wearable electrofluidic actuation system, which exploits the alternating current electrothermal (ACET) effects to engineer biofluid flow profiles on the body. The wearable ACET flow is induced with the aid of corrosion-resistant electrode configurations (fabricated on a flexible substrate) and custom-developed, wirelessly programmable high frequency (MHz) excitation circuitry. Various tunable flow profiles are demonstrated with the aid of the devised flexible ACET electrode configurations, where the induced profiles are in agreement with the ACET theory and simulation. The demonstrated capabilities rendered by the presented system create new degrees of freedom for implementing advanced bioanalytical operations for future lab-on-the-body platforms.

A rapid and low-cost fabrication and integration scheme to render 3D microfluidic architectures for wearable biofluid sampling, manipulation, and sensing

The large-scale deployment of wearable bioanalytical devices for general population longitudinal monitoring necessitates rapid and high throughput manufacturing-amenable fabrication schemes that render disposable, low-cost, and mechanically flexible microfluidic modules capable of performing a variety of bioanalytical operations within a compact footprint. The spatial constraints of previously reported wearable bioanalytical devices (with microfluidic operations confined to 2D), their lack of biofluid manipulation capability, and the complex and low-throughput nature of their fabrication process inherently limit the diversity and frequency of end-point assessments and prevent their deployment at large scale. Here, we devise a simple, scalable, and low-cost "CAD-to-3D Device" fabrication and integration scheme, which renders 3D and complex microfluidic architectures capable of performing biofluid sampling, manipulation, and sensing. The devised scheme is based on laser-cutting of tape-based substrates, which can be programmed at the software-level to rapidly define microfluidic features such as a biofluid collection interface, microchannels, and VIAs (vertical interconnect access), followed by the vertical assembly of pre-patterned layers to realize the final device. To inform the utility of our fabrication scheme, we demonstrated three representative devices to perform sweat collection (with visualizable secretion profile), sample filtration, and simultaneous biofluid actuation and sensing (using a sandwiched-interface). Our devised scheme can be adapted for the fabrication and manufacturing of current and future wearable bioanalytical devices, which in turn will catalyze the large-scale production and deployment of such devices for general population health monitoring.

Wearable Electrochemical Microneedle Sensor for Continuous Monitoring of Levodopa: Toward Parkinson Management

Levodopa is the most effective medication for treating Parkinson's disease (PD). However, because dose optimization is currently based on patients' report of symptoms, which are difficult for patients to describe, the management of PD is challenging. We report on a microneedle sensing platform for continuous minimally invasive orthogonal electrochemical monitoring of levodopa (L-Dopa). The new multimodal microneedle sensing platform relies on parallel simultaneous independent enzymatic-amperometric and nonenzymatic voltammetric detection of L-Dopa using different microneedles on the same sensor array patch. Such real-time orthogonal L-Dopa sensing offers a built-in redundancy and enhances the information content of the microneedle sensor arrays. This is accomplished by rapid detection of L-Dopa using square-wave voltammetry and chronoamperometry at unmodified and tyrosinase-modified carbon-paste microneedle electrodes, respectively. The new wearable microneedle sensor device displays an attractive analytical performance with the enzymatic and nonenzymatic L-Dopa microneedle sensors offering different dimensions of information while displaying high sensitivity (with a low detection limit), high selectivity in the presence of potential interferences, and good stability in artificial interstitial fluid (ISF). The attractive analytical performance and potential wearable applications of the microneedle sensor array have been demonstrated in a skin-mimicking phantom gel as well as upon penetration through mice skin. The design and attractive analytical performance of the new orthogonal wearable microneedle sensor array hold considerable promise for reliable, continuous, minimally invasive monitoring of L-Dopa in the ISF toward optimizing the dosing regimen of the drug and effective management of Parkinson disease.

Physiology of sweat gland function: The roles of sweating and sweat composition in human health

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

Graphene-Based Sensors for Human Health Monitoring

Since the desire for real-time human health monitoring as well as seamless human-machine interaction is increasing rapidly, plenty of research efforts have been made to investigate wearable sensors and implantable devices in recent years. As a novel 2D material, graphene has aroused a boom in the field of sensor research around the world due to its advantages in mechanical, thermal, and electrical properties. Numerous graphene-based sensors used for human health monitoring have been reported, including wearable sensors, as well as implantable devices, which can realize the real-time measurement of body temperature, heart rate, pulse oxygenation, respiration rate, blood pressure, blood glucose, electrocardiogram signal, electromyogram signal, and electroencephalograph signal, etc. Herein, as a review of the latest graphene-based sensors for health monitoring, their novel structures, sensing mechanisms, technological innovations, components for sensor systems and potential challenges will be discussed and outlined.